SOS induction by thermosensitive replication mutants of miniF plasmid

Area of Science:

• Molecular Biology
• Genetics

Background:

• The F plasmid's miniF fragment (9.3 kb) contains genes essential for replication, partition, and SOS signal expression.
• The SOS response is a cellular defense mechanism triggered by DNA damage or replication stress.

Purpose of the Study:

• To investigate the conditions and genetic requirements for SOS induction by miniF plasmid replication arrest.
• To elucidate the role of the miniF lynA locus and host genes (recA, lexA) in this process.

Main Methods:

• Utilizing thermosensitive miniF plasmids (miniFts) with mutations affecting replication.
• Inducing SOS functions by arresting replication at 42°C and observing effects on prophage lambda and sfiA expression.
• Analyzing the impact of incompatibility genes (incBC+) and plasmid integration (pR325) on SOS induction.
• Assessing the requirement for miniF lynA+, host recA+, and lexA+ genes.

Main Results:

• Arrest of miniFts replication at 42°C induced SOS functions, including prophage lambda induction and sfiA expression.
• SOS induction was dependent on the miniF lynA+ locus in cis and host recA+ and lexA+ genes.
• Blocking miniF replication via trans-acting incBC+ genes also induced SOS functions.
• SOS induction was heightened near the stationary phase, correlating with decreased cell viability.
• miniFts plasmids were rapidly lost during host cell exponential growth, yet SOS induction persisted.

Conclusions:

• The arrest of miniF plasmid replication is a potent trigger for the SOS response.
• The lynA gene product of miniF is crucial for SOS induction upon replication arrest.
• SOS induction is modulated by growth phase and can persist independently of plasmid presence.
• A persistent lynA product is hypothesized to contribute to chromosomal DNA unwinding, potentially explaining persistent SOS induction.